1. Field of the Invention
This invention relates to spinal implant devices and methods for promoting fusion between adjacent vertebral bodies, and more particularly to expandable fusion devices that can be inserted between adjacent vertebral bodies to facilitate the fusion thereof.
2. Description of the Relevant Art
The human spine is a complex mechanical structure, composed of alternating bony vertebrae and fibrocartilaginous discs that are connected by strong ligaments and supported by musculature, that extends from the skull to the pelvis and provides axial support for the body.
The vertebrae generally comprise a vertebral foramen bounded by the anterior vertebral body and the neural arch. The vertebral body comprises two end plates (i.e., superior and inferior) made of thin cartilage overlying a thin layer of hard cortical bone that attaches to the spongy, cancellous interior bone of the vertebral body. The neural arch consists of two pedicles and two lamina that are united posteriorly. The spinous and transverse processes protrude from the neural arch. The superior and inferior articular facets lie at the root of the transverse processes.
The intervertebral discs primarily serve as a mechanical cushion between adjacent vertebral segments of the spinal column and generally comprise two basic components: the annulus fibrosis and the nucleus pulposus. The annulus fibrosis forms the outer perimeter of the disc and is a tough ring that binds adjacent vertebrae together. The nucleus pulposus fills the interior of the disc and carries load.
The spine as a whole is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. However, genetic or developmental irregularities, trauma, chronic stress, and degenerative wear can result in spinal pathologies for which surgical intervention may be necessary.
It is common practice to remove a spinal disc in cases of spinal disc deterioration, disease or spinal injury. More particularly, the discs sometimes become diseased or damaged such that the height of the disc is reduced, which causes the annulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the annulus begin to buckle and separate, circumferential and/or radial annular tears may occur, allowing nucleus material to escape or form a bulge in the annulus. Such disruption to the natural intervertebral separation and the resulting herniation produces pain, which can be alleviated by removal of the disc and restoration of the natural separation distance. In cases of chronic back or leg pain resulting from a degenerated or herniated disc, removal of the disc can become the desired course of treatment.
In some cases it is desired to fuse the adjacent vertebrae together after removal of the disc. Such a procedure is sometimes referred to as “intervertebral fusion” or “interbody fusion”.
Many techniques and instruments have been devised to perform intervertebral fusion. There is common agreement that the strongest intervertebral fusion is interbody fusion between the lumbar bodies, which may be augmented by a posterior or facet fusion. In cases of intervertebral fusion, either structural bone, or a rigid interbody fusion “cage” typically filled with morselized bone, is placed centrally within the space where the spinal disc once resided. Multiple bony grafts or cages may be used within that space. Furthermore, multiple surgical approaches may be utilized, including anterior, posterior, or lateral surgical approaches.
Such practices are characterized by certain disadvantages, including the need to distract the disc space in order to implant the fusion device and thereby restore the diseased disc space to its normal or healthy height. However, it can be difficult to distract the adjacent vertebral bodies sufficiently to easily insert the fusion device between adjacent vertebral bodies. As a result, it is often necessary to drive the fusion device into the space between the vertebral bodies using impaction with a mallet and the application of significant force. The use of such impaction and force increases the risk of damage to local soft tissue such as blood vessels and the surrounding nerves, and can lead to suboptimal placement and/or failure of the insertion instrumentation. Furthermore, the use of such impaction and force can damage or compromise the vertebral endplates, resulting in eventual failure and subsidence of the fusion device into the vertebral bodies and hence loss of disc height.
Therefore, there is a need for a fusion device that can be placed between adjacent vertebral bodies at minimal height and, thereafter, be variably adjusted with minimal force application to the preferred height for an individual patient. Furthermore, it is desirable that the expandable fusion device be maintained in a closed (i.e., unexpanded) position during insertion and handling, and that it be rigidly attachable to a holder so as to facilitate maximum control by the surgeon during insertion and deployment.